1. Field of the Invention
This invention relates to a process for the production of materials resembling suede, using coating compounds which may be cross-linked to high molecular weight products at elevated temperatures and which, by virtue of the particular variation of the viscosity thereof with temperature, are particularly suitable for the reversal coating of matrices which have exceptionally deep and narrow depressions. The sheets obtained after removal of the coating from the matrix have long fibers projecting from the surface so that they are particularly suitable for use as suede imitations.
2. Description of the Prior Art
It is known to coat various types of matrices with thermoplastic resins by spraying or by application using a doctor knife and subsequently removing the coatings from the matrix after evaporation of any solvents used or, in the case of reactive systems, after the hardening reaction. The thus-obtained coatings carry a negative reproduction of the surface of the matrix.
Suitable matrix materials include, for example, the conventional embossed separating papers having surfaces of, for example, polypropylene or silicone, metals, silicone rubber compounds or polyester resin compounds.
The coating systems used are preferably thermoplastic compounds, polyurethanes dissolved in solvents and capable of being cross-linked, e.g. with polyisocyanates or melamine resins, PVC compounds which may contain varying amounts of plasticizer, polyamides, etc. (U.S. Pat. No. 3,861,937).
Blocked two-component reactive systems have also been described for the conventional coatings, for example, in U.S. Pat. No. 3,755,261, there is described the amine-hardening of polymers or prepolymers based on urethanes having isocyanate end groups, on epoxy resins or on halogen-containing hydrocarbon polymers, etc., using finely divided complexes of certain aromatic diamines and salts, such as NaCl.
Isocyanate prepolymers blocked with the conventional blocking agents are also known, e.g., according to German Offenlegungsschrift No. 2,131,299 (U.S. Pat. No. 3,770,703), isocyanate prepolymers blocked with caprolactam, or prepolymers blocked with malonic dialkyl esters. Ketoxime-blocked isocyanate prepolymers which are subsequently cross-linked with polyols containing tertiary amine groups have also been described in German Offenlegungsschrift No. 1,621,910 for the advantageous characteristics thereof compared with phenol-blocked isocyanate prepolymers.
According to German Offenlegungsschrift No. 1,519,432 (U.S. Pat. No. 3,267,078), ketoxime-blocked isocyanate prepolymers are mixed with polyketimines (i.e., polyamines blocked with ketones) which harden at room temperature in the presence of moisture.
In British Pat. No. 1,085,454, there are described ketoxime-blocked isocyanate prepolymers with aliphatic diamines for the production of abrasion-resistant coatings or impregnations of nonwovens, leather and wood.
Although these described reactive systems are generally known in the use thereof for the production of coatings, they have not been described for the production of imitation suedes nor are the particular conditions required for producing such imitation suedes known. Solvent-free coatings are obtained from foils of thermoplastic resins which soften when heated so that they fill up the depressions in the matrices. The polymer solidifies on cooling so that a negative reproduction of the matrix is obtained on the surface of the foil.
The matrices described are generally used to imitate surfaces of the type commonly found in grained leather, although they are, of course, also able to reproduce other patterns such as textile structures, etc. It is particularly difficult, however, to produce suede imitations by these means. Matrices which have been taken from genuine suede or from flocked suede-like materials are distinguished by having exceptionally narrow and deep depressions which enable the long hairs or fibers of a suede to be reproduced. In U.S. Pat. No. 3,533,895, foils of soluble thermoplastic polyurethane elastomers are impressed into silicone matrices at high temperatures and pressures. The products, however, must not be separated from the matrix until they have cooled and the nap thereof is very sensitive to pressure for from 1 to 2 days, as well as the products being very sensitive to solvents.
So it was tried to get an improved thermostability by aftertreatment of such soluble polyurethane leathers by spraying with diluted solutions of polymer isocyanates, drying at 80.degree. C. and standing three days at room temperature (German utility model DGBM No. 7 141 380). However, this procedure is complicated and lengthy and, due to the use of solvents, results in environmental pollution.
The production of matrices and the use thereof in reversal coating processes is known. They have been described, for example, in U.S. Pat. Nos. 3,369,949; 3,655,497 and 4,124,428 and in German Offenlegungsschrift No. 1,933,255. One difficulty of the process, however, is that high molecular weight thermoplastic products generally do not enter the narrow depressions of such matrices sufficiently completely. If, on the other hand, low viscosity solutions of polymers are used, these solutions will readily enter the deep depressions, but faulty areas are formed as the solvent evaporates so that the attractive character of the suede is not perfectly reproduced. These faulty areas are subsequently fixed by the increase in viscosity with resulting film formation which occurs at the same time.
Similarly unsatisfactory results are obtained from the reaction of deblocked polyurethane reactive systems, even if they have a high solids content, e.g., of 67% by weight and, here again, the formation of a nap is impaired and the structure of the polyurethane suede is not sufficiently stable for removal from the matrix. The polyurethane obviously fails to penetrate sufficiently completely into the fine capillaries and fine velour hairs stick and break off as the polyurethane is separated from the matrix.
On the basis of these results, it was to be expected that high solids reactive systems obtained from blocked starting materials would give rise to similar difficulties on separation of the reactive polyurethanes after they have hardened inside the fine capillaries of the matrix.
It has surprisingly been found, however, that if the conditions according to the present invention are observed, the systems chosen according to the present invention are not only capable of reproducing the finest details in the matrix (such as very fine suede hairs) but also enable the polyurethane urea to be readily stripped from the matrix after polyurethane formation from the reactive systems has been completed within the matrix. These systems are based on blocked polyurethane starting components which do not harden to a solvent-resistant polyurethane urea until heated and are preferably cross-linked after the heat treatment. In addition to the detailed and accurate reproduction of the matrix and the ease with which the product may be stripped from the matrix, the procedure according to the present invention ensures that the back of the polyurethane suede material is homogeneous and of uniform thickness.
However, before the onset of cross-linking, if the viscosity minimum within the temperature range of 30.degree. to 80.degree. C. is not observed, but instead, for example, the temperature is immediately raised to high furnace temperatures such as 170.degree. C., the same materials will give rise to unhomogeneous suede coatings full of blisters and crater-like depressions.